The benefit of combined electric and acoustic stimulation (EAS) for speech and pitch perception has been demonstrated in a number of previous studies. In some cases, EAS benefit has been documented even when cochlear-implant (CI) patients have very limited residual hearing and speech perception ability in the non-implanted ear. To date, it is still unclear how individual differences in sensory inputs, linguisti context, and cognitive functions influence the degree of benefit provided by EAS, and it is not known whether the typical EAS patient utilizes their residual hearing to its greatest potential. These uncertainties limit clinicians' and patients' ability to make good decisions related to second-ear implantation. In this research, we seek to identify factors that underlie EAS benefit and to investigate methods that could potentially enhance the benefits of residual hearing in EAS users. Unlike the descriptive approach employed by most previous studies, we will take a more comprehensive, model-based approach that considers both the bottom-up and top-down processes that contribute to multi-source speech perception in EAS users. Aim 1 will determine how EAS benefit is influenced by listeners' ability to utilize and optimally weight speech cues presented to the CI and residual hearing ears. Aim 2 will investigate how bottom-up low-frequency acoustic cues and top-down processing (such as the use of linguistic context and the ability to fill in missing speech information) interact to improve speech intelligibility in EAS usrs. Finally, Aim 3 will develop and test speech-enhancement algorithms that are likely to improve speech perception by EAS users. Overall, this research should add substantially to our understanding of 1) the degree of benefit that can be expected from low-frequency residual hearing in EAS, 2) the mechanisms responsible for EAS benefit and the factors that account for its variability across individuals, and 3) the nature of signal-processing algorithms that may enhance speech perception in EAS users.